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| Titel |
Component geochronology of the ca. 3920 Ma Acasta Gneiss |
| VerfasserIn |
Nicole L. Cates, Stephen J. Mojzsis, Guillaume Caro, Michelle D. Hopkins, Oleg Abramov, Dustin Trail, Wouter Bleeker, Martin Guitreau, Janne Blichert-Toft |
| Konferenz |
EGU General Assembly 2013
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 15 (2013) |
| Datensatznummer |
250077565
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| Zusammenfassung |
| Compiled U-Pb zircon ages of the oldest parts of the Acasta Gneiss Complex (AGC) in the
Northwest Territories (Canada) span about 4050-3850 Ma (Stern and Bleeker, 1998); yet
older 4200 Ma xenocrystic U-Pb zircon ages have also been reported for this terrane (Iizuka
et al., 2006). The AGC has at least 50 km2 of outcrop exposure, but only a small subset has
been documented in the detail required to investigate a complex history. To better understand
this history, ion microprobe zircon geochronology was combined with whole-rock and zircon
rare earth element compositions (+Y; [REE+Y]zirc) and Ti-in-zircon thermometry (Tixln)
from a sub-divided ~60 cm2 slab of Acasta banded gneiss, and compared to other nearby
variably deformed AGC granitoid gneiss samples. Micro-sampling by this method
reveals components with distinctive [Th/U]zirc vs. Tixln and [REE+Y]zirc that are
correlative with separate 235,238U-207,206Pb zircon age populations and whole-rock
compositions, but not with 147Sm-143Nd isotope systematics. Lattice-strain theory
used to model [REE+Y] reconciles U-Pb zircon geochronology for the individual
components, which also preserve strong positive Eu* anomalies. Modeling shows that
the magmas that gave rise to the oldest domains formed at contemporary oxygen
fugacities. The AGC preserves a legacy older than about 4000 Ma, but this derives from
incomplete assimilation of older crust. Magmatic emplacement at ca. 3920 Ma is
contemporaneous with the Late Heavy Bombardment (LHB) of the Moon. Later
superimposed Eoarchean events (3850-3720 Ma) are reminiscent of formation times for the
Itsaq Gneiss Complex in West Greenland (Nutman et al., 1996), Nuvvuagittuq
Supracrustal Belt in northern Québec (Cates et al. 2013), and Manfred Complex in
Western Australia (Kinny et al., 1990). Equilibration of Sm-Nd occurred at the
scale of individual components over the course of one or more of these events. |
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